The general objective is to characterize the transport function of individual nephron segments in order better to understand kidney function. Renal tubules are dissected from rabbit kidneys and perfused in vitro. In the present studies bicarbonate transport by proximal convoluted tubules was measured, using a newly developed micro calorimeter to assay total CO2 content. The pincipal conclusion is that bicarbonate reabsorption is dependent on simultaneous transport of sodium.